1. Field of the Invention
The present invention relates to an exhaust device of an eight-cylinder engine.
2. Description of the Related Art
One conventional exhaust device of a multicylinder engine is disclosed in Japanese Patent Document JP-A-2000-265836, in which one set of exhaust passages extends respectively from a plurality of cylinders subjected to odd-numbered explosions and another set of exhaust passages extends respectively from a plurality of cylinders subjected to even-numbered explosions. These passages are joined individually at one point, and further joined at another point into one exhaust passage. With this structure, exhausts from the cylinders subjected to explosion in serial order are less likely to directly interfering with each other.
However, if, as in the above document, the engine is used as a drive source for an outboard motor, the length of the exhaust passages may be shortened to make the overall engine more compact. In such a case, cylinders subjected to odd-numbered explosions, which occur prior, and the cylinders subjected to even-numbered explosions, which occur later and subsequently to the former, are positioned in proximity to each other because of the length of the shortened exhaust passages described above.
As a result, exhausts from the cylinders subjected to later explosions tend to interfere with exhausts from the cylinders subjected to prior explosions. Such interference tends to decrease the intensity of desired exhaust pulses that generate negative pressure, and sufficient negative pressure may not be obtained.
When the negative pressure of exhaust pulses is not sufficiently high, the exhaust is released very effectively from the cylinders. This can cause knocking due to the burnt gas left in the cylinders, misfiring, increased pumping losses, and decreased volumetric efficiency due to an improper intake of fresh air. As a result, engine output, fuel economy and exhaust efficiency may decrease.